The use of silicon in semiconductor devices, such as Floating-Gate Avalanche-injection MOS transistors ("FAMOS") and Floating-Gate tunnel-oxide transistor ("FLOTOX") is well known. Equally well known is the time dependent degradation of these devices, which is often referred to as the hot carrier degradation effect. Typically, the FAMOS and FLOTOX structures have to withstand many program and erase operations, which is usually at least 10.sup.4 cycles. After so many program and erase operations, the threshold voltage window (i.e., the difference after program and erase) decreases due to the deterioration of the oxide quality in terms of interface traps, injection efficiency, and leakage.
It is believed that the interface traps are caused by defects that are generated by current flow in such semiconductor devices, and it is further believed that these defect states reduce the mobility and lifetime of the carriers and cause degradation of the device's performance. In most cases, the substrate comprises silicon, and the defects are thought to be caused by dangling bonds (i.e., unsaturated silicon bonds) that introduce states in the energy gap, which remove charge carriers or add unwanted charge carriers in the device, depending in part on the applied bias. While dangling bonds occur primarily at surfaces or interfaces in the device, they also are thought to occur at vacancies, micropores and dislocations, and are also thought to be associated with impurities. To alleviate the problems caused by such dangling bonds, a hydrogen passivation process has been adopted and has become a well-known and established practice in the fabrication of such devices.
In the hydrogen passivation process, it is thought that the defects that affect the operation of semiconductor devices are removed when the hydrogen bonds with the silicon at the dangling bond sites. While the hydrogen passivation process eliminates the immediate problem associated with these dangling bonds, it does not eliminate degradation permanently because the hydrogen atoms that are added by the passivation process can be "desorbed" or removed from the previous dangling bond sites by the "hot carrier effect."
A hot carrier is an electron or hole that has a high kinetic energy, which is imparted to it when voltages are applied to electrodes of the device. Under such operating conditions, the hydrogen atoms, which were added by the hydrogen passivation process, are knocked off by the hot electrons. This hydrogen desorption results in aging or degradation of the device's performance. According to established theory, this aging process occurs as the result of hot carriers stimulating the desorption of hydrogen from the silicon substrate's surface or the silicon dioxide interface. This hot carrier effect is particularly of concern with respect to smaller devices in which proportionally larger electric fields can be used.
Accordingly, what is needed in the art is a semiconductor device and a method of manufacture therefore that does experience the level of efficiency degradation experienced by the devices that are passivated with conventional hydrogen passivation processes. The present invention addresses these needs.